Polymer-pen lithography (PPL) is a molecular printing technology that combines elements of dip-pen nanolithography and microcontact printing to print arbitrary patterns with nanoscale registration between features. See Huo et al., 321 Science 1658-60 (2008); Piner et al., 283 Science 661-63 (1999); Ginger et al., 43 Angew. Chem. Int. Ed. 30-45 (2004); Salaita et al., 2 Nat. Nanotech. 145-55 (2007); Kumar et al., 63 Appl. Phys. Lett. 2002-04 (1993); Xia et al., 99 G. M. Chem. Rev. 1823-48 (1999); Xia et al., 7 Adv. Mater. 471-73 (1995); and Wilbur et al., 7 Adv. Mater. 649-52 (1995). PPL has the ability to print features ranging in edge length from about 90 nm to tens of micrometers in a single writing operation. PPL offers several advantages over DPN, including higher throughput by using a massively parallel array of elastomeric tips (for example up to 107 tips) and the ability to change the feature size using either contact force or dwell time. The ability to vary the feature size by varying the contact force between the tip array and the substrate is a feature unique to PPL and occurs because the elastomeric tips deform upon contact with the surface, thus increasing the contact area and in turn, the feature edge length. Recently, a quantitative relationship between the force between the tip array and the substrate and the resulting feature edge lengths was established and verified experimentally.
A challenge that arose in PPL was obtaining precise leveling between the planes of the pen array and the substrate because it was found that misalignment between the planes of the pen array and the substrate resulted in pens across the array not being in contact with the surface simultaneously and the pens not being deformed identically. Previously, optical methods were used to judge the alignment between the two planes. The optical method included pressing the pens into the surface, and when it appeared that all pens deformed by the same amount when viewed through a microscope, the arrays were judged to be level with respect to the surface. This optical leveling method was only able to level the two planes within 0.01°, which can lead to a large deviation in feature size across a 1×1 cm pen array.
Recently it was determined that by maximizing the force between the pen array and the substrate upon a given z-piezo extension, the two planes could be leveled within 1×10−4 degree. Using this force feedback leveling strategy, features of 16-mercaptohexadecanoic acid were written on a gold surface with a size variance of less than 2% over a distance of 1 cm. See Liao et al., 10 Nano Lett. 1335-40 (2010). Leveling the array with respect to the substrate uses the previously established relationship between z-piezo extension and force. See Liao et al., 6 Small 1082-85 (2009).